#define _CRT_SECURE_NO_WARNINGS 1

class Solution
{
public:
	ListNode* swapPairs(ListNode* head)
	{
		if (head == nullptr || head->next == nullptr) return head;
		auto tmp = swapPairs(head->next->next);
		auto ret = head->next;
		head->next->next = head;
		head->next = tmp;
		return ret;
	}
};